Proposed in 1905 by Thomas C. Chamberlin and Forest R. Moulton as an alternative to the nineteenth‑century Laplacian nebular model, the Chamberlin–Moulton hypothesis sought to explain Solar System formation through a tidal interaction between the young Sun and a passing star. In the original formulation a close stellar encounter produced tidal bulges on the solar surface and, aided by solar eruptive activity, repeatedly expelled material that was stretched by the passerby’s gravity into two spiral‑like arms. Most of this matter would fall back to the Sun, but a fraction was envisioned to remain in bound orbits where it would cool and solidify into a population of small bodies—“planetesimals”—and a smaller number of protoplanets. Planet formation then proceeded by collisional accretion among these solids, with remaining debris becoming asteroids and comets.
Early observational and theoretical evidence seemed to favor the idea: photographs of “spiral nebulae” were interpreted as possible analogues of the hypothesized ejection‑and‑condensation process (these nebulae are now identified as distant galaxies). Subsequent refinements and critiques altered and ultimately undermined the model. In 1917 J. H. Jeans argued that a sufficiently close stellar passage alone could strip solar material, obviating the need to invoke prominences. More decisive objections followed: in 1939 Lyman Spitzer showed that an extended gaseous filament drawn from the Sun would tend to dissipate rather than fragment into solids, and in the 1940s H. N. Russell demonstrated that material removed with the energy required to account for Jupiter’s angular momentum would likely escape the solar system rather than settle into planetary orbits. These dynamical and condensation problems led to the abandonment of the Chamberlin–Moulton hypothesis as a complete formation theory. Its enduring legacy is the central role of planetesimals—small solid precursors that accrete to form larger bodies—which remains a foundational concept in modern models of planetary formation. (Source material carrying this historical summary included a maintenance note requesting additional citations as of October 2009.)